There are several key factors to a successful centerless grinding operation, including:
- Clearly understanding the expected production requirements for the job.
- Determining the grindability of the work material (wear-resistant materials take longer to process).
- Understanding how metal-removal rate (MRR) has a direct impact on part surface finish and overall part tolerances.
- Properly setting up the machine. A proper setup makes life infinitely easier for the manufacturing engineer and production team.
Coolant and Coolant Delivery
Coolant is a key component to successful centerless grinding but is often overlooked. Many coolant delivery systems for centerless grinders are under engineered and are often neglected.
The basic functions of grinding fluids are to:
- Prevent distortion or metallurgical damage to the part by holding the temperature steady. In order to maintain a uniform temperature, a steady, measured flow of coolant is required.
- Act as a lubricator, reducing the friction between the wheel and the workpiece.
- Wash away chips of metal and abrasive particles which would otherwise mar the workpiece surface and load the grinding wheel face.
- Keep the work blade and guides clean and well lubricated.
Oil is typically the best coolant, and it is recommended to use premium, ester-based, high-performance grinding oil. However, oil may not always be practical, so water-soluble and synthetic coolants may be required in some applications.
Coolant velocity must match wheel velocity. For an untested grind cycle, use 25 gpm/inch. If power is known, use 1.5-2.0 gpm/hp where practical.
The coolant nozzle should provide a coherent flow of coolant. This means the coolant coming out of the nozzle looks like a solid bar, void of any air entrapment. This can be accomplished through correctly sized supply pipes with a limited number of bends in the coolant lines. Lastly, a properly designed nozzle will help reduce turbulence and air entrapment.
Maintain coolant at ambient temperature whenever possible. Tank size should be able to satisfy coolant flow without interruption of coolant during a continuous grind cycle. Filtration is necessary to eliminate reprocessing grinding swarf.
The goal of dressing the grinding wheel is to make the wheel round and “flat.” At the same time, dressing exposes new sharp grains to assist in the cutting action of the abrasive. Stationary dressing tools are still very common. If your machine is equipped for stationary tooling, make certain to use a high-quality dress nib. Relatively quick, light passes are most effective, and 8-12 ipm @ 0.0005″-0.001″ depth of cut is a good place to start.
Rotary dressing systems are the best option for production centerless grinding. They provide a more consistent process and allow for more abrasive selections, such as high ceramic concentration and superabrasive wheels.
High-quality, hand-set diamond dressing rolls with synthetic reinforcing logs are the best choice. The roll and wheels should run in the same direction at point of contact – “unidirectional.” For peak sharpness of the wheel, run the diamond roll at 80% of grinding wheel speed measured in sfpm. Traversing the diamond roll across the wheel at 6-12 ipm provides a sharp, aggressive wheel. The dress comp should not exceed 0.001″.
Dressing the proper shape into the feed wheel is critical. On through-feed operations, the feed wheel is often tilted 3° to help pull the part through the grinding zone. It’s critical to maintain a straight line of contact between the grinding wheel, the part and the feed wheel. For this reason, machine builders provide diamond offsets to assist in putting a specific shape into the feed wheel. It typically ends up with a dog bone shape. A single-point U-Dex-IT tool is used to allow for measured tool rotation to extend diamond life. Typical feeds are 5-10 ipm for rough grinding and 1-2 ipm for finish grinding. Depth of cut is typically 0.001″.
There is a lot of helpful information on centerless grinding available. If you need assistance, contact the Norton| Saint-Gobain Abrasives Application Engineering Team to improve your centerless grinding process or visit www.nortonabrasives.com, choose your region and search for centerless grinding.
About the author: Phil Plainte is senior applications engineer for Norton | Saint-Gobain Abrasives, Worcester, MA